Always add 360 , PS3 , or PC to clarify which system the bug appears on.

PC 360 Re-entering the Glenmoril Cave during Blood's Honor makes the Witch Heads droppable. During the Purity quest, in which you must attempt the removal of Farkas and Vilkas's Lycanthropy, you must have one Glenmoril Witch head in your inventory. When the quest item glitch is initiated the NPC's do not recognize them in your inventory and make the quest impossible to complete. To resolve this, simply have your chosen NPC follow you during any of their side quests. Before completion of said quest, go inside of Glenmoril Witch Cave. Finish the quest, re-speak to them and the Purity quest activates, permanently turning all of the Witch heads in your inventory to disposable.

360 If dropped in one of the Dremora in the shrine of Merhrunes Dagon, a dremora will always drop them.

PC 360 PS3 When doing the Purity quest the Glenmoril Witch Heads may not be consumed when the fire is activated. The quest can still be completed.

PC Sometimes the head will not show up in the inventory.

PC(Fix) The console command to add a Glenmoril Witch Head to the player's inventory is: player.additem 0002996f x where x = the number of witch heads to add.

PS3 After all quests related to the witch heads have been completed, it should be possible to drop them. If you do and then pick them up again they will sometimes be stuck in inventory as essential quest items and be impossible to drop.